[0001] This invention relates to a method of protecting, reinforcing, or strengthening a
catalyst or catalyst precursor, especially a Fischer catalyst or catalyst precursor.
The invention further relates to strengthened catalysts and catalyst precursors thus
obtained, and to processes in which such strengthened catalysts and catalyst precursors
are used.
[0002] The Fischer-Tropsch process can be used for the conversion of hydrocarbonaceous feed
stocks into liquid and/or solid hydrocarbons. The feed stock (e.g. natural gas, associated
gas and/or coal-bed methane, coal) is converted in a first step into a mixture of
hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas or
syngas). The synthesis gas is then converted in one or more steps over a suitable
catalyst at elevated temperature and pressure into paraffinic compounds ranging from
methane to high molecular weight molecules comprising up to 200 carbon atoms, or,
under particular circumstances, even more.
[0003] Numerous types of reactor systems have been developed for carrying out the Fischer-Tropsch
reaction. For example, Fischer-Tropsch reactor systems include fixed bed reactors,
especially multi tubular fixed bed reactors, fluidised bed reactors, such as entrained
fluidised bed reactors and fixed fluidised bed reactors, and slurry bed reactors such
as three-phase slurry bubble columns, moving bed reactors, and ebullated bed reactors.
[0004] The Fischer-Tropsch reaction is very exothermic and temperature sensitive with the
result that careful temperature control is required to maintain optimum operation
conditions and desired hydrocarbon product selectivity. Bearing in mind the very high
heat of reaction which characterises the Fischer-Tropsch reaction, the heat transfer
characteristics and cooling mechanisms of a reactor are important.
[0005] Three-phase slurry bubble column reactors potentially offer advantages over the fixed-bed
design in terms of heat transfer performance. Such reactors typically incorporate
small catalyst particles in a liquid continuous matrix. The synthesis gas is bubbled
through, maintaining suspension of small catalyst particles and providing the reactants.
The motion of the continuous liquid matrix promotes heat transfer to achieve a high
commercial productivity. The catalyst particles are moving within a liquid continuous
phase, resulting in efficient transfer of heat generated by the catalyst particles
to the cooling surfaces. The large liquid inventory in the reactor provides a high
thermal inertia, which helps prevent rapid temperature increases that can lead to
thermal runaway.
[0006] The micron-sized catalyst particles must be removed from the reaction products, as
at least part of the reaction products are in the liquid phase under reactor conditions.
Because of the small size of the particles this separation is difficult, and is typically
carried out using expensive internal or external filtration system. Other issues associated
with the use of suspended catalyst particles are non-uniform distribution of catalyst
throughout the reactor (with knock-on effects on cooling) and catalyst attrition.
[0007] The current invention relates to catalysts and catalyst precursors suitable for use
in a slurry reactor and having a longest internal straight length of at least 1 mm
and having a highly porous structure. Such catalysts and catalyst precursors can be
delicate and thus difficult to handle.
[0008] Broken catalyst particles, fines and dust may be formed during transport of the porous
particles. Also when installing such porous particles into a reactor broken catalyst
particles, fines and dust may be formed. Very small catalyst particles are undesired
because they may cause handling problems, and they may end up in the product. As discussed
above, small sized particles are difficult to separate from the product.
[0009] It is thus desired to protect, reinforce, or strengthen catalysts or catalyst precursors
that are highly porous and are having a size of at least 1 mm before these are transported
and/or before loading into the reactor. Particles having a particle size of at least
1mm are defined as particles having a longest internal straight length of at least
1mm.
[0010] Accordingly, the present invention provides a method of strengthening a catalyst
precursor comprising a porous body having a longest internal straight length of at
least 1 mm and the porous body along with carrier material having a porosity of at
least 50% and having pores with a size of more than 10 µm, the method comprising at
least the following step:
- (a) before application of one or more catalytically active components or precursors
therefor to the catalyst precursor, adding one or more waxes to the catalyst precursor.
[0011] Further, the present invention provides a method of strengthening a catalyst or catalyst
precursor comprising a porous body having a longest internal straight length of at
least 1 mm and the porous body along with catalyst or catalyst precursor material
having a porosity of at least 50% and having pores with a size of more than 10 µm,
the method comprising at least the following step:
- (a) before use of the catalyst or catalyst precursor, adding one or more waxes to
the catalyst or catalyst precursor.
[0012] The porous bodies act as support for the catalyst/catalyst precursor material that
is located thereon. The combined porous bodies and catalyst or catalyst precursor
material will be referred to as "catalyst bodies". The catalyst or catalyst precursor
material comprises a carrier and a catalytically active component or precursor therefor.
The or each catalytically active component may be in its/their oxidised state whilst
one or more waxes are added. The catalyst or catalyst precursor may then be reduced
in
situ, i.e. in the reactor, before use. The wax is typically provided over or within pores
of the catalyst bodies, that is over the catalyst or catalyst precursor material,
which is itself typically over or within the porous bodies. The use of the catalyst
or catalyst precursor is defined as its use as catalyst, for example as catalyst for
a Fischer Tropsch reaction. Preferably the catalyst is a Fischer-Tropsch catalyst.
[0013] As well as reducing dust formation and partially (at least) protecting the catalyst
from oxidation during reactor set-up, the wax facilitates the mounting of the catalyst
within the reactor by reducing the friction between the internal components of the
reactor (such as the cooling tubes or the edge of the reactor) and the catalyst bodies
and can further help mechanically strengthen the catalyst.
[0014] The porous bodies may be of regular or irregular shapes, or a mixture thereof. Such
include cylinders, cubes, spheres, ovoids, etc, and other shaped polygons. In general,
"size" can be considered as their longest internal straight length.
[0015] In a preferred embodiment the porous bodies have a form or shape selected from the
group consisting of gauze, honeycomb, monolith, sponge, mesh, webbing, foil construct
and woven mat form, or any combination of these.
[0016] The porous bodies may be a combination of forms such as those listed above. For example,
the porous bodies may be made up of honeycomb shaped material and have a circular
outer shape. Another example is a cylinder made from woven mat.
[0017] The porous bodies may suitably be made from refractory oxides; for example titania
(TiO
2), silica (SiO
2), alumina; metals, for example stainless steel, iron or copper; or any similar inert
material capable of withstanding conditions within the reactor.
[0018] The porosity within the porous bodies, i.e. the internal voidage of the porous bodies
before application of the catalyst or catalyst precursor material on the porous bodies,
is within the range 50-95%; preferably the internal voidage is more than 60%, more
preferably more than 70%, even more preferably more than 80%, and most preferably
more than 90% (with reference to the circumferential volume of the bodies). Before
application of the catalyst or catalyst precursor material on the porous bodies, the
porosity within the porous bodies may be even up to 98%.
[0019] The porosity of the catalyst body (i.e. including the catalyst or catalyst precursor
material and the porous body) is at least 50% and is preferably at least 65%, more
preferably around 85%.
[0020] The external voidage of the catalyst bodies, i.e. the porous bodies on which the
catalyst has been applied, in situ in the reactor is between 5-60%, preferably less
than 40% by volume, more preferably about 20% by volume.
[0021] The open volume within the catalyst bodies must be sufficient to facilitate efficient
through-flow of reactants, while at the same time the specific surface area of each
catalyst body should be as large as possible to increase exposure of reactants to
the catalyst material. The open nature of the catalyst bodies allow the same or similar
catalyst loading to be achieved as with prior micron-sized catalyst particles, such
there is no reduction of the catalyst activity and STY by use of bigger catalyst bodies.
[0022] Suitable porous bodies, on which the catalyst or catalyst precursor material can
be applied, can be prepared in-house or alternatively be obtained commercially. An
example of a producer of suitable porous bodies is the Fraunhofer-Institute for Manufacturing
and Advanced Materials in Dresden, Germany. The Fraunhofer-Institute advertises and
sells, for example, melt extracted metallic fibres, and highly porous fibre structures
that can be cylindrically or spherically shaped.
[0023] The catalyst or catalyst precursor material may be applied to the porous bodies as
a thin layer. The catalyst or catalyst precursor material layer is preferably sufficiently
thin to avoid diffusional mass transport limitation (decrease of CO and/or hydrogen
partial pressure and/or unfavourable change of the hydrogen/carbon monoxide-ratio
within the catalyst layer) of the syngas components within the catalyst layer. Thickness
of the catalyst layer can be increased up to the onset of mass transport limitation.
There is no upper limit to the thickness of the catalyst layer onto the porous bodies
other than mass transport limitation and voidage of the substrate for hydrodynamic
reasons.
[0024] It is preferred that the catalyst or catalyst precursor material fraction of the
catalyst bodies is at least about 1% by volume and preferably greater than about 4%
by volume (with reference to the volume of the catalyst bodies), with a preferred
maximum of 25% by volume.
[0025] Preferably the catalyst or catalyst precursor material is applied as a layer to the
porous bodies, typically in a thickness of from about 1 to about 300 microns and preferably
from about 5 to about 200 microns.
[0026] General methods of preparing catalyst or catalyst precursor materials are known in
the art, see for example
US 4409131,
US 5783607,
US 5502019,
WO 0176734,
CA 1166655,
US 5863856 and
US 5783604. These include preparation by co-precipitation and impregnation. Such processes could
also include freezing, sudden temperature changing, etc. Control of the component
ratio in the solid solution can be provided by parameters such as residence time,
temperature control, concentration of each component, etc.
[0027] The catalyst or catalyst precursor material, generally based on a catalytically active
metal, may be present with one or more metals or metal oxides as promoters, more particularly
one or more d-metals or d-metal oxides.
[0028] Preferably the catalyst is a Fischer-Tropsch catalyst. Fischer-Tropsch catalysts
are known in the art, and typically include a Group 8-10 metal component, preferably
cobalt, iron and/or ruthenium, more preferably cobalt.
[0029] Suitable metal oxide promoters may be selected from Groups 2-7 of the Periodic Table
of Elements, or the actinides and lanthanides. In particular, oxides of magnesium,
calcium, strontium, barium, scandium, yttrium, lanthanum, cerium, titanium, zirconium,
hafnium, thorium, uranium, vanadium, chromium and manganese are most suitable promoters.
[0030] Suitable metal promoters may be selected from Groups 7-10 of the Periodic Table.
Manganese, iron, rhenium and Group 8-10 noble metals are particularly suitable, with
platinum and palladium being especially preferred. The amount of promoter present
in the catalyst is suitably in the range of from 0.01 to 100 pbw, preferably 0.1 to
40, more preferably 1 to 20 pbw, per 100 pbw of carrier.
[0031] Preferred noble metals are platinum, palladium, rhodium, ruthenium, iridium and osmium.
[0032] References to "Groups" and the Periodic Table as used herein relate to the new IUPAC
version of the Periodic Table of Elements such as that described in the 87
th Edition of the Handbook of Chemistry and Physics (CRC Press).
[0033] Any promoter(s) is typically present in an amount of from 0.1 to 60 parts by weight
per 100 parts by weight of a porous carrier. It will however be appreciated that the
optimum amount of promoter(s) may vary for the respective elements which act as promoter(s).
If the catalyst comprises cobalt as the catalytically active metal and manganese and/or
vanadium as promoter, the cobalt: (manganese + vanadium) atomic ratio is advantageously
between 5:1-30:1.
[0034] In one embodiment of the present invention, the catalyst or catalyst precursor comprises
the promoter(s) and/or co-catalyst(s) having a concentration in the Group 8-10 metal(s)
in the range 1-10 atom%, preferably 3-7 atom%, and more preferably 4-6 atom%.
[0035] Typically the catalyst or catalyst precursor material comprises a carrier material
such as a porous inorganic refractory oxide, preferably alumina, silica, titania,
zirconia or mixtures thereof. The most preferred refractory oxide carrier material
is titania. The carrier could be added onto the porous bodies prior to addition of
the catalytically active metal by impregnation for example. Alternatively, the catalytically
active metal and carrier material could be admixed and then added to the porous bodies.
For example, a powder form of the catalyst material could be formed into a slurry,
and then spray coated onto the porous bodies.
[0036] A suitable catalyst comprises cobalt as the catalytically active metal and zirconium
as a promoter. Another suitable catalyst comprises cobalt as the catalytically active
metal and manganese and/or vanadium as a promoter.
[0037] In a preferred embodiment, when preparing catalyst bodies use is made of porous bodies
of which more than 95%, more preferably more than 99%, most preferably 100%, has a
size of 1-50 mm, preferably 1-30 mm.
[0038] Catalyst bodies comprising porous bodies with a minimum size of 1 mm, and a maximum
size of up to 50 mm, may be able to be supported by the slurry, and are therefore
movable within the reactor vessel so as to seek the most even catalytic transfer and
heat transfer, but without being fixed within the reactor.
[0039] In an alternative embodiment, the catalyst bodies are larger, for example up to 500
mm, even up to 2m, and may be immobilised within a reactor. For example, larger catalyst
bodies in an immobilised slurry reactor may be fixed between cooling tubes.
[0040] In a method according to the invention, typically the catalyst body is at least partly
coated in said waxes. The waxes may be present in pores within the catalyst body,
especially in the small pores of the carrier material on the porous body, as well
as on the surface of the catalyst body.
[0041] Preferably, the wax is substantially non-tacky below a temperature of about 40 °C.
[0042] The wax may include esters, ethers, alcohols, paraffins and alpha olefins. One preferred
wax comprises a C
20+ alpha olefin or paraffin, more preferably a C
20-100 alpha paraffin.
[0043] Other examples of waxes suitable for use in the present invention include natural
waxes such as petroleum waxes, e.g., paraffin waxes and hydrocarbon waxes; fatty acid
oil, beeswax; mineral waxes, e.g., microcrystalline montan wax; vegetable waxes such
as carnauba wax and synthetic waxes such as polyethylene, polypropylene, polymethylene,
chemically modified waxes, and polymerized alpha-olefins and more especially Fischer-Tropsch
waxes.
[0044] One advantage of using Fischer Tropsch waxes when the catalyst is a Fischer Tropsch
catalyst, is that the Fischer Tropsch catalyst and reactor is designed for contact
with such waxes during the reaction and so their use to coat the catalyst before use
is less likely to poison the catalyst or reactor system compared to adding foreign
waxes.
[0045] Preferably the wax is a hydrocarbon wax, more preferably a branched hydrocarbon wax
having a melting point ranging from 40°C to 90°C, preferably from 50°C to 80°C, and
more preferably 55°C to 75°C.
[0046] Preferably therefore at least one of the waxes, preferably all of the waxes are liquid
at a temperature of above 60°C.
[0047] Typically the wax comprises at least 30 wt% paraffinic molecules, more typically
50 wt% paraffinic molecules, preferably more than 70 wt% especially more than 90 wt%.
[0048] The wax may be at least 95 wt% C
10-C
100 .
[0049] Typically the wax is liquid at a temperature of more than 40°C, preferably more than
60°C, especially more than 80°C.
[0050] Polyethylene waxes suitable for use in the coating composition of the invention are
represented by the following general formula: (-CH
2CH
2-)
n. Polyethylene waxes having the above general formula are commercially available from
Baker Petrolite under the trademark PETROLITE(R). Preferred polyethylene waxes are
those having an average molecular weight ranging from about 650 to about 30,000 and
a particle size ranging between 1 micron and 140 microns.
[0051] To protect the catalyst form the atmosphere only a surface coating is sufficient.
In that case only the surface of the catalyst body is coated with wax, for example
using spray coating. To provide good beneficial mechanical stability preferably at
least 90% of the small pores in the carrier on the porous body are filled with wax.
It is also possible to fill up to 90%, or even 100% of the large pores of the catalyst
body, i.e. the pores with a size of more than 10 µm. In that case the volume of the
wax is larger than the volume of the catalyst body in the wax covered catalyst body.
[0052] Preferably the or each wax fills more than 10% of the pore volume of the carrier
on the porous body, typically more than 30%, preferably more than 50%, more preferably
more than 70%. The amount of wax deposited varies depending on the technique used
to deposit the wax. For example spraying the wax onto the carrier on the porous body
can typically result in less than 30% of the small pores in the carrier being filled
with wax whilst dipping the catalyst body into wax can result in almost 100% of the
small pores in the carrier on the porous body being filled with wax.
[0053] In a preferred embodiment, the wax is applied at the exterior of the catalyst body
so that the wax can act as lubricant when installing the catalyst body into the reactor.
This is especially advantageous when a relatively large, preferably with a longest
internal straight length larger than 50 mm and smaller than 2 m, and optionally just
fitting, catalyst body is installed between cooler tubes in a reactor.
[0054] The or each wax in step (a) may be added to the catalyst or catalyst precursor by
spraying, dipping, coating, impregnating and soaking; preferably dipping.
[0055] Preferably the or each wax of step (a) is added in liquid form at a temperature of
< 250°C, preferably around 200°C.
[0056] Preferably a step (b) is performed following step (a), step (b) comprising absorbing
any superfluous wax with an absorbent material.
[0057] Optionally a step (c) is performed following step (a), and when step (b) is performed,
also following step (b); step (c) comprising heating the catalyst to a temperature
sufficient to melt the wax.
[0058] Preferably step (a) is performed outside the reactor in which the catalyst is used.
[0059] The invention also provides a method for loading a catalyst or catalyst precursor
into a reactor, the method comprising:
- strengthening the catalyst or catalyst precursor using a method as defined herein;
- loading the catalyst or catalyst precursor into a reactor;
- removing at least some of the wax from the catalyst;
- using the catalyst.
[0060] Typically all of the wax is removed from the catalyst before using the catalyst.
[0061] Where a catalyst precursor is loaded into the reactor, this typically comprises an
oxidised form of the active metal and thus typically the catalyst precursor is then
reduced before use.
[0062] An advantage of certain embodiments of the present invention is the reduced amount
of dust formation when loading the catalyst into the reactor.
[0063] A further advantage of certain embodiments of the invention is the stability of the
catalyst - normally it is required to provide the active metal of a Fischer Tropsch
catalyst in its oxidised form to prevent reaction of the active metal and the formation
of unwanted cobalt compounds. Thus the metal oxide in the catalyst is normally reduced
in situ to the pure metal. For certain embodiments of the invention the active metal may
be provided in its less oxidised or even metallic form since the wax coating prevents
or minimises the reaction of the active metal.
[0064] A further advantage of certain embodiments of the invention is that the wax layer
provides a protective coating on the catalyst body which increases its mechanical
stability thus making it more resistant to physical shocks and thus less likely to
break during transport, handling and loading.
EXAMPLES
[0065] Embodiments of the present invention will now be described, by way of example only,
with reference to the accompanying figures in which:
Fig. 1 is a side sectional view showing a process for adding wax to a catalyst body;
Fig. 2 is a side sectional view of a mould and catalyst structure used to add wax
to the catalyst body;
Fig 3 is a top view of the Fig. 2 mould and catalyst body.
[0066] As shown in Fig. 1 a cylindrical mould 10a, 10b, 10c is provided into which a catalyst
body 12 is placed. The mould in turn is placed in a vessel 14.
[0067] In this embodiment, the catalyst body 12 comprises a metal gauze having catalyst
or catalyst precursor material disposed thereon, suitable to catalyse a Fischer Tropsch
reaction.
[0068] Wax, which has been heated slightly to reduce its viscosity, is added to the catalyst
body 12 and allowed to set. A coolant, such as solid carbon dioxide 16, is added to
the vessel 14 and increases the rate of wax-setting and also allows the wax to set
from the outside to the inside of catalyst body 12.
[0069] As shown in Fig. 2, a bottom 10a of the mould is removed and liquid wax is allowed
to drain away from the gauze structure leaving a wax layer 18 towards the outside
of the catalyst body12. Clearly the thickness of the layer 18 can be varied depending
on the residence time of the mould in the coolant or other variables. For certain
embodiments the wax may be allowed to completely set and so no wax is drained away.
[0070] This procedure was conducted a number of times for catalyst structures having a density
of (i) 400g/1 and (ii) 200g/l. The resulting wax weight is detailed in table 1 below.
Table 1
Structure Density |
Draining after (min) |
Weight Wax (g) |
400g/l |
1 |
114 |
400g/l |
3 |
188 |
400g/l |
not drained |
374 |
200g/l |
1 |
69 |
200g/l |
3 |
102 |
200g/l |
12 |
324 |
200g/l |
not drained |
344 |
[0071] The catalyst structure 12 may be placed in a slurry reactor, depending on the size
of the catalyst structure a fixed slurry bed may be obtained. A particular benefit
is that the wax provides a lubricant to allow the catalyst structure 12 to be slotted
into the reactor and also increases the mechanical strength of the catalyst, which
for embodiments which do not include a metal structure, is particularly beneficial.
[0072] Moreover, the wax can reduce the dust formation when loading and can also reduce
the risk of oxidation with the air.
[0073] Improvements and modifications may be made without departing from the scope of the
invention.
1. A method of strengthening a catalyst precursor, said catalyst precursor comprising:
- carrier material, and
- a porous body
said porous body having a longest internal straight length of at least 1 mm and the
porous body along with carrier material having a porosity of at least 50% and having
pores with a size of more than 10 µm, the method comprising at least the following
step:
(a) before application of one or more catalytically active components or precursors
therefor to the catalyst precursor, adding one or more waxes to the catalyst precursor.
2. A method of strengthening a catalyst or catalyst precursor, said catalyst or catalyst
precursor comprising:
- catalyst or catalyst precursor material, and
- a porous body;
said porous body having a longest internal straight length of at least 1 mm and the
porous body along with catalyst or catalyst precursor material having a porosity of
at least 50% and having pores with a size of more than 10 µm, the method comprising
at least the following step:
(a) before use of the catalyst or catalyst precursor, adding one or more waxes to
the catalyst or catalyst precursor.
3. A method as claimed in claim 2 wherein the catalyst or catalyst precursor is a Fischer
Tropsch catalyst or precursor therefore suitable for a slurry reactor.
4. A method as claimed in any preceding claim wherein at least one of the waxes of step
(a), preferably all of the waxes of (a) are liquid at a temperature of above 40°C,
preferably above 60°C, more preferably above 80°C.
5. A method as claimed in either preceding claim, wherein the one or more waxes comprise
a C20+ alpha olefin or paraffin, preferably a C20-100 alpha paraffin.
6. A method as claimed in any preceding claim, wherein the wax comprises at least 30%
paraffinic molecules.
7. A method as claimed in any preceding claim, wherein the porous bodies have a form
or shape selected from the group consisting of gauze, honeycomb, monolith, sponge,
mesh, webbing, foil construct and woven mat form, or any combination of these.
8. A method as claimed in any preceding claim, wherein the porous bodies comprise metal
porous bodies, preferably aluminium or stainless steel gauze.
9. A method as claimed in any one of claims 2-8, wherein the catalyst or catalyst precursor
material comprises a porous inorganic refractory oxide, such as alumina, silica, titania,
zirconia or mixtures thereof especially titania as carrier material.
10. A method for transporting catalyst or catalyst precursor particles, characterised in that the particles are first treated according to a method according to any one of claims
1 to 9.
11. A method for installing catalyst particles or catalyst precursor particles in a reactor,
characterised in that the particles are first treated according to a method according to any one of claims
2 to 9.